Bernie Sanders has vowed to stay in the Democratic presidential nomination race until the end after winning Oregon's primary and battling Hillary Clinton to a razor-thin margin in Kentucky.
The primary in Kentucky is too close to call with Mrs Clinton leading Mr Sanders by less than 0.5%. But Mrs Clinton declared victory in the primary, telling her supporters on Twitter: "We're always stronger united."
Donald Trump won the Republican Party's Oregon primary. His campaign announced it had signed a joint fundraising agreement with the Republican National Committee that will allow it to raise cash for both his campaign and other Republican efforts.
Democrats displayed new signs that it could have trouble uniting around Mrs Clinton's candidacy as Mr Sanders vowed to press on until the end of the primary calendar in mid-June. He will need to win about two-thirds of the remaining pledged delegates to end the primary season in a tie, but is not giving up.
Mr Sanders told a rally in Carson, California: "Before we will have the opportunity to defeat Donald Trump, we're going to have to defeat secretary Clinton."
Mrs Clinton ended the night with a commanding lead of 279 pledged delegates over Mr Sanders and a dominant advantage among party officials and elected leaders known as superdelegates.
The outcomes in Kentucky and Oregon, where Mr Sanders led by 9 percentage points with roughly three-quarters of the vote counted, did not dramatically change the delegate count. The former US secretary of state remains on track to clinch the nomination on June 7 in the New Jersey primary.
But Tuesday's elections followed a divisive weekend state party convention in Nevada in which supporters of Mr Sanders were accused of throwing chairs and making death threats against the Nevada party chairwoman at the event in Las Vegas. Supporters argued that party leadership had rigged the results of the convention in favour of Mrs Clinton.
In a sign of the tensions between the two sides, Mr Sanders issued a defiant statement on Tuesday dismissing complaints from Nevada Democrats as "nonsense" and said his supporters were not being treated with "fairness and respect".
Later, in California, Mr Sanders said the party could "do the right thing and welcome into the party people who are prepared to fight for real economic and social change". He said the other option would be to "maintain its status quo structure, remain dependent on big-money campaign contributions and be a party with limited participation and limited energy".
Mr Sanders pointed to polls that showed he fares more favourably than Mrs Clinton in a head-to-head with Mr Trump. With his win in Oregon, Mr Trump now has 1,160 delegates, putting him within 77 delegates of clinching the Republican nomination.
Mr Trump had 67% of the vote in Oregon with about a quarter of the vote left to be counted. Texas senator Ted Cruz and Ohio governor John Kasich both got about 16% of the vote.
For Democrats, Mrs Clinton and Mr Sanders will each pick up at least 25 delegates in Kentucky with five delegates remaining to be allocated pending final vote tallies. In Oregon, Mr Sanders will receive at least 28 delegates and Mrs Clinton will get at least 24 of the 61 delegates at stake.
Overall, Mrs Clinton leads Mr Sanders among pledged delegates with 1,767 to his 1,488. When superdelegates are included, Mrs Clinton's lead grows to 2,291 to Mr Sanders' 1,528. Mrs Clinton is now just 92 delegates short of the 2,383 needed to win.
The Sanders campaign did not immediately say whether it will challenge the results in Kentucky, which does not have an automatic recount.
Mrs Clinton campaigned in Kentucky on Sunday and Monday in an effort to halt Mr Sanders' momentum after his recent victories in Indiana and West Virginia. She pointed to the economic gains under the administration of her husband, former US president Bill Clinton, who is the last Democrat to carry the state in a presidential election.
Nearing the end of a long primary slog, the two Democratic candidates are preparing for June 7 primaries in California, New Jersey and four other states and then the District of Columbia primary on June 14.
When pledged delegates and superdelegates are combined, Mrs Clinton is now about 96% of the way towards securing the Democratic nomination.